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dude

Sixty Cycle Blues

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Very cool. I'm not going to use any active filter controls (just the variable negative feedback knob), so that's okay with me. Though if I have room when I'm punching holes tomorrow, I'll just put in a PPIMV and a regular MV.

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How important is the value of the grid resistors on the EL34s? Since I'm cathode-biasing my power tubes, I'm thinking I can probably just replace the 220k grid resistors that normally go between the grid and the negative bias voltage supply with a dual-gang 220k (or higher) pot for my PPIMV. Is that okay or is the grid current pretty big on these beam power pentodes?

Also, for my 6au6 preamp stage: I'm thinking I'll run the first of the two bootstrapped as a triode, and then run the second with a high plate voltage in pentode mode (mostly because this is what's provided on the datasheet). My theory is that I'll get a bunch of gain from the first stage that will distort on the relatively low-gain second stage and give me some nice, tasty pentode distortion. That might be totally wrong though.

EDIT:

Actually its probably safe to run them both as pentodes. Datasheet shows the transconductance getting higher with lower grid#2 voltages (more "pentode-ey" operation). Granted I'll have a lower plate resistor, which will reduce the gain and max voltage swing somewhat, but I ought to be fine, I think.

Edited by dude

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How important is the value of the grid resistors on the EL34s? Since I'm cathode-biasing my power tubes, I'm thinking I can probably just replace the 220k grid resistors that normally go between the grid and the negative bias voltage supply with a dual-gang 220k (or higher) pot for my PPIMV. Is that okay or is the grid current pretty big on these beam power pentodes?

Your EL34s are cathode biased - there is no dedicated negative bias supply other than what is generated across the cathode resistors.

Subbing in your PPIMV pot in place of the 220k grid resistors will be fine. I'd also recommend grid stopper resistors in series with the EL34 grids, around 4-5k (may be suggested in your datasheet?).

The schematic in the 5th post down the following link is a good starting point. Ignore the 1M resistors in parallel with the PPIMV pot, just sub in your 250k dual pot and grid stoppers in place of the 8.2k resistors.

http://ampgarage.com/forum/viewtopic.php?p=24181&sid=4b78ef45476054b9051dcb2bce745763

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Awesome, that's more-or-less what I was thinking (minus the grid stoppers, so thanks for that).

Alright, I should be able to order some parts today. Hopefully I can get started soldering this thing up later this week.

Thanks for all your help.

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